1. Field of the Invention
The present invention relates to an area lighting device and a liquid crystal display device having the same, particularly to an area lighting device using an LED as a light source and a liquid crystal display device having the same.
2. Description of the Related Art
A liquid crystal display device has a liquid crystal display panel provided with a plurality of pixels arranged in a matrix, and a backlight unit which illuminates light from behind the liquid crystal display panel. The liquid crystal display device drives liquid crystals to control the transmittance of light from the backlight unit at each of the pixels for display. In recent years, in order to expand the color reproduction range (color gamut) of the liquid crystal display device, it is studied to use an LED for the light source of the backlight unit, becoming practical use. In using a plurality of the LEDs, it is necessary to control the balance of the light quantity of each LED because the properties of LEDs are different from one another. Particularly, when LEDs that emit single color lights of red (R), green (G), and blue (B) are combined for use, it is required to control the balance of the light quantity of each of red, green and blue LEDs.
For that scheme, there is a technique in which an optical sensor is disposed on one part of a backlight unit (for example, on the back side of a light guide plate), the optical sensor senses the chromaticity or brightness of light combined with each color of R, G and B, each of the LEDs is feedback controlled based on the sensed chromaticity or brightness, and the balance of the quantity of R, G and B lights is optimized. The optical sensor senses the chromaticity and brightness of the combined lights, and outputs signals depending thereon. The LEDs are driven based on the signals to control the light emission brightness of each of the LEDs, and to control the chromaticity and brightness of light emitted from the backlight unit.
Patent Reference 1: JP-A-2004-29141
To a viewer who views the display screen of a liquid crystal display device, light enters the viewer's eyes which has come out of a backlight unit and passed through a liquid crystal display panel. The optical property of the liquid crystal display panel is changed because of changes in the wavelength property of liquid crystals due to temperature and aging (for example, deterioration in color filters and aging of the wavelength property of liquid crystals). Therefore, since changes in the optical property of the liquid crystal display panel are not reflected even though the chromaticity and brightness of light emitted from the backlight unit is controlled as described above, the display chromaticity and display brightness of the liquid crystals display device are sometimes varied.
FIG. 9 is a graph illustrating the variation over time in display chromaticity in which almost white is displayed on the display screen of a liquid crystal display device of related art. The horizontal axis of the graph depicts the lighting time (minute) of LEDs of a backlight unit, and the vertical axis depicts the display chromaticity of the liquid crystal display device. A line connecting black circles depicts a chromaticity x, and a line connecting white circles depicts a chromaticity y. As shown in FIG. 9, the display chromaticity of the liquid crystal display device is relatively greatly varied right after the LEDs are lighted, and it takes a long time until it becomes stable. As described above, the liquid crystal display device of related art has a problem that display chromaticity and display brightness are varied over time after the LEDs are lighted and display quality is not stabilized.